Various types of clutches have been used to transmit torque in vehicles. In a vehicle having an automatic transmission, for example, multiple clutches are used to engage or disengage selected gearsets to obtain a desired gear ratio. The clutches provided for this purpose are typically friction plate clutches, which include a plurality of inner or drive plates that are splined to a rotatable hub. A housing or case is coaxially aligned with the hub and supported for rotation independent of the hub. A plurality of outer or driven plates are splined to the housing or case and are interposed between the drive plates, such that the drive and driven plates are alternately arranged on a common axis. The drive plates include a friction material on one or both sides of the plates. An actuator, such as a hydraulic piston, applies a pressure force that presses the drive and driven plates into engagement, thereby permitting torque to be transferred from the hub to the housing.
The number of plates used in a friction plate clutch is typically selected to transmit a maximum desired torque. Due to space constraints and a maximum pressure force that can be generated by the actuator, higher torque transfer is typically achieved by providing additional plates. As a result, friction plate clutches commonly use several plates to meet the maximum torque design. When the clutch is unengaged or idling, however, the plates generate a parasitic drag torque due to the viscous shearing action which exists between the drive and driven plates as they rotate at different speeds relative to one another. This drag torque adversely affects vehicle fuel economy for automatic transmissions, and therefore the use of excessive plates exacerbates this problem.
The use of numerous friction plates also reduces the controllability of the friction clutch in the lower torque range. The relationship between actuator pressure and the amount of torque transfer produced by a friction plate clutch is substantially linear, as illustrated by the dashed line 4 in FIG. 1. For a given increase in actuator pressure, the increase in torque is relatively large when a large number of friction plates are used. This results in a relatively steep incline as shown by the curve 4, which indicates a high sensitivity of the clutch to pressure. As noted above, the actuator is typically a hydraulic actuator that uses hydraulic fluid to generate the actuator pressure. The hydraulic fluid flows through a hydraulic valve, such as a solenoid valve, having a controller operably coupled thereto. The increased sensitivity to actuator pressure exhibited by conventional friction plate clutches makes it more difficult to control the hydraulic valve to obtain the desired torque.
Synchronizing clutches are known that employ both friction plates and a dog or mechanical connection. U.S. Pat. No. 6,966,805 to Caldwell, for example, discloses a marine transmission having a friction mechanism to at least partially synchronize the rotational speed of a driving shaft and a driven shaft prior to engagement of a dog clutch element. While the friction mechanism transfers some torque, it only begins to rotate the driven shaft so that it is sufficiently synchronized with the driving shaft, and is not intended to operate over any appreciable portion of the operational torque range of the transmission.